The present invention is directed to television receivers, and particularly to synchronous detection systems for use in such receivers.
Television receivers of the type considered herein include a synchronous detector which receives a 45 megahertz oscillator signal and a 45 megahertz video I.F. signal for synchronously demodulating the composite video signal. The oscillator signal is typically derived by applying the video I.F. signal to a limiter which is manually tuned to provide optimum differential phase response. The output of the limiter is coupled to a phase locked loop in which a phase detector receives the oscillator output of a VCO (voltage controlled oscillator) and the output of the limiter to develop an error signal which locks the frequency of the VCO to the frequency of the video I.F. signal. The oscillator signal thus developed is applied to one input of the synchronous detector.
To ensure that the VCO input to the synchronous detector and the video input thereto are at a selected phase with respect to each other (zero degrees, for example), a manually operable phase adjustment network may be included in the phase locked loop. By appropriate adjustment of this network the phase of the VCO input to the synchronous detector may be adjusted so that it is precisely in phase with the video input.
Although the arrangement described above performs satisfactorily, manufacturing costs can be reduced if the phase adjustment network is eliminated and the limiter is manually tuned to provide the required phase shift for the VCO signal input to the synchronous detector. If that option is selected, tuning the limiter to achieve the proper phase of the VCO signal may result in sub-standard differential phase response in the limiter. The present invention overcomes this problem in a manner that permits the limiter to be adjusted for best differential phase response and yet eliminates the cost associated with manually adjusting the phase adjustment network.